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<h2><span class="refentrytitle">Macro BOOST_TTI_TRAIT_HAS_TEMPLATE</span></h2>
<p>BOOST_TTI_TRAIT_HAS_TEMPLATE — A macro which expands to a metafunction which tests whether an inner class template with a particular name exists. </p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="header/boost/tti/has_template_hpp.html" title="Header &lt;boost/tti/has_template.hpp&gt;">boost/tti/has_template.hpp</a>&gt;

</span>BOOST_TTI_TRAIT_HAS_TEMPLATE(trait, ...)</pre></div>
<div class="refsect1">
<a name="idm45674780369616"></a><h2>Description</h2>
<p>BOOST_TTI_TRAIT_HAS_TEMPLATE is a macro which expands to a metafunction. The metafunction tests whether an inner class template with a particular name exists. The macro takes the form of BOOST_TTI_TRAIT_HAS_TEMPLATE(trait,...) where</p>
<p>trait = the name of the metafunction <br>
 ... = variadic parameters.</p>
<p>The first variadic parameter is the inner class template name.</p>
<p>Following variadic parameters are optional.</p>
<p>If no following variadic parameters exist, then the inner class template being introspected must be all template type parameters ( template parameters starting with <code class="computeroutput">class</code> or <code class="computeroutput">typename</code> ) and any number of template type parameters can occur.</p>
<p>If the second variadic parameter is BOOST_PP_NIL and no other variadic parameter is given, then just as in the previous case the inner class template being introspected must be all template type parameters ( template parameters starting with <code class="computeroutput">class</code> or <code class="computeroutput">typename</code> ) and any number of template type parameters can occur. This form is allowed in order to be consistent with using the non-variadic form of this macro.</p>
<p>If the second variadic parameter is a Boost preprocessor library array and no other variadic parameter is given, then the inner class template must have its template parameters matching the sequence in the tuple portion of the Boost PP array. This form is allowed in order to be consistent with using the non-variadic form of this macro.</p>
<p>Otherwise the inner class template must have its template parameters matching the sequence of the optional variadic parameters.</p>
<p>BOOST_TTI_TRAIT_HAS_TEMPLATE generates a metafunction called "trait" where 'trait' is the first macro parameter.</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">BOOST_TTI_TP_T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">trait</span>
  <span class="special">{</span>
  <span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">unspecified</span><span class="special">;</span>
  <span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">bool_</span><span class="special">&lt;</span><span class="keyword">true</span><span class="special">-</span><span class="identifier">or</span><span class="special">-</span><span class="keyword">false</span><span class="special">&gt;</span> <span class="identifier">type</span><span class="special">;</span>
  <span class="special">}</span><span class="special">;</span>

<span class="identifier">The</span> <span class="identifier">metafunction</span> <span class="identifier">types</span> <span class="identifier">and</span> <span class="keyword">return</span><span class="special">:</span>

  <span class="identifier">BOOST_TTI_TP_T</span> <span class="special">=</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">in</span> <span class="identifier">which</span> <span class="identifier">to</span> <span class="identifier">look</span> <span class="keyword">for</span> <span class="identifier">our</span> <span class="char">'name'</span><span class="special">.</span>
                   <span class="identifier">The</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">can</span> <span class="identifier">be</span> <span class="identifier">a</span> <span class="keyword">class</span><span class="special">,</span> <span class="keyword">struct</span><span class="special">,</span> <span class="identifier">or</span> <span class="keyword">union</span><span class="special">.</span>
  
  <span class="identifier">returns</span> <span class="special">=</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">the</span> <span class="char">'name'</span> <span class="keyword">template</span> <span class="identifier">exists</span> <span class="identifier">within</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span><span class="special">,</span>
            <span class="identifier">otherwise</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">false</span><span class="special">.</span>
</pre>
<p>Examples:</p>
<pre class="programlisting"><span class="number">1</span><span class="special">)</span> <span class="identifier">Search</span> <span class="keyword">for</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">class</span> <span class="keyword">template</span> <span class="identifier">called</span> <span class="char">'MyTemplate'</span><span class="special">,</span> <span class="identifier">with</span> <span class="identifier">all</span> <span class="keyword">template</span> <span class="identifier">type</span> <span class="identifier">parameters</span><span class="special">,</span>
   <span class="identifier">nested</span> <span class="identifier">within</span> <span class="identifier">the</span> <span class="keyword">class</span> <span class="char">'MyClass'</span> <span class="keyword">using</span> <span class="identifier">a</span> <span class="identifier">metafunction</span> <span class="identifier">name</span> <span class="identifier">of</span> <span class="char">'MyMeta'</span><span class="special">.</span>

   <span class="identifier">BOOST_TTI_TRAIT_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">MyMeta</span><span class="special">,</span><span class="identifier">MyTemplate</span><span class="special">)</span>

   <span class="identifier">or</span>

   <span class="identifier">BOOST_TTI_TRAIT_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">MyMeta</span><span class="special">,</span><span class="identifier">MyTemplate</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span> <span class="comment">// Non-variadic macro form</span>

   <span class="identifier">MyMeta</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">value</span>

   <span class="identifier">is</span> <span class="identifier">a</span> <span class="identifier">compile</span> <span class="identifier">time</span> <span class="identifier">boolean</span> <span class="identifier">constant</span> <span class="identifier">which</span> <span class="identifier">is</span> <span class="identifier">either</span> <span class="char">'true'</span> <span class="identifier">or</span> <span class="char">'false'</span>
   <span class="keyword">if</span> <span class="identifier">the</span> <span class="identifier">nested</span> <span class="keyword">template</span> <span class="identifier">exists</span><span class="special">.</span>

<span class="number">2</span><span class="special">)</span> <span class="identifier">Search</span> <span class="keyword">for</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">class</span> <span class="keyword">template</span> <span class="identifier">called</span> <span class="char">'MyTemplate'</span><span class="special">,</span> <span class="identifier">with</span> <span class="keyword">template</span> <span class="identifier">parameters</span> 
   <span class="identifier">of</span> <span class="char">'class T,int x,template&lt;class&gt; class U'</span><span class="special">,</span> <span class="identifier">nested</span> <span class="identifier">within</span> <span class="identifier">the</span> <span class="keyword">class</span> <span class="char">'MyClass'</span> 
   <span class="keyword">using</span> <span class="identifier">a</span> <span class="identifier">metafunction</span> <span class="identifier">name</span> <span class="identifier">of</span> <span class="char">'MyMeta'</span><span class="special">.</span>

   <span class="identifier">BOOST_TTI_TRAIT_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">MyMeta</span><span class="special">,</span><span class="identifier">MyTemplate</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span><span class="special">)</span>

   <span class="identifier">or</span>

   <span class="identifier">BOOST_TTI_TRAIT_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">MyMeta</span><span class="special">,</span><span class="identifier">MyTemplate</span><span class="special">,</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="special">(</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span><span class="special">)</span><span class="special">)</span><span class="special">)</span> <span class="comment">// Non-variadic macro form</span>

   <span class="identifier">MyMeta</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">value</span>

   <span class="identifier">is</span> <span class="identifier">a</span> <span class="identifier">compile</span> <span class="identifier">time</span> <span class="identifier">boolean</span> <span class="identifier">constant</span> <span class="identifier">which</span> <span class="identifier">is</span> <span class="identifier">either</span> <span class="char">'true'</span> <span class="identifier">or</span> <span class="char">'false'</span>
   <span class="keyword">if</span> <span class="identifier">the</span> <span class="identifier">nested</span> <span class="keyword">template</span> <span class="identifier">exists</span><span class="special">.</span>
</pre>
<p> </p>
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